Effect of (H2O)n (n = 1–3) clusters on H2O2 + HO → HO2 + H2O reaction in tropospheric conditions: competition between one-step and stepwise routes

Abstract

ABSTRACTEffects of (H2O)n (n = 1–3) on the H2O2 + HO → HO2 + H2O reaction have been investigated by the reactions of H2O2L(H2O)n (n = 1–3) + HO and H2O2 + HOL(H2O)n (n = 1–3) at the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ level of theory, coupled with rate constant calculations by using canonical variational transition state theory. Interestingly, for the former reactions, one-step process and stepwise mechanism are involved, where one-step processes occurring though cage-like hydrogen bonding network complexes and the transition states are favourable. Due to larger effective rate constants, these favourable processes are also favourable than the corresponding latter reactions. Meanwhile, the catalytic effect of (H2O)n (n = 1–3) is mainly taken from water monomer, because the effective rate constant (k'(R_WM2)) of H2O2···H2O + HO reaction is, respectively, larger by 3, 6–10 orders of magnitude than that of H2O2···(H2O)2 + HO (k'(R_WD1)) and H2O2···(H2O)3 + HO (k'(R_WT1)) reactions. Furthermore, the enhancement factor of water molecular (k'(R_WM2)/ktot) is only 0.28% at 240 K, while at high temperature (such as at 425 K), the positive water vapour effect enhances up to 27.13%. This shows that at high temperatures the positive water effect is obvious under atmospheric conditions.